The goal of this project is to study molecular structures of actomyosin interaction involved in the process of force generation in skeletal muscle. The myosin heads, namely the cross-bridges, have shown to generate force in the direction perpendicular to fiber axis, when form crosslinks between the myosin filament and the actin filament. The radial force is a function of the filament separation, i.e. the attached cross-bridges are elastic in the radial direction. Based on our data of cross-bridges in various strong binding states, we have previously proposed that the radial elastic behavior depends on the state of the cross-bridges. In FY92 we obtained data indicating that the radial elasticity of the cross-bridges bound weakly to actin is different from those bound strongly to actin, providing further support to the proposal. The significance of the finding is that the dependence of radial elasticity of the state provides a simple and direct way of differentiating structures of attached cross-bridges. The effects of strongly bound myosin fragment S1 on regulation of muscle contraction have been investigated. At low Ca++ level, S1 increases the Ca++ sensitivity while at high CA++ level, the maximal force level is suppressed. The time course of force re-development is affected by the strongly bound S1. The results suggest that strongly bound cross-bridges can activate muscle by modulating kinetics of force production.